The present invention generally relates to maintenance signal transmission systems, and more particularly to a system for transmitting a maintenance signal by switching a maintenance signal transmission line in a transmission system.
There are transmission systems having a backup line in addition to a main line. In most cases, the main and backup lines coexist in the same optical fiber cable, and this arrangement is sufficient for the purposes of coping with a failure of a communication unit. However, when the optical fiber cable breaks, both the main and backup lines fail and it is impossible to overcome the failure by switching the line from the main line to the backup line. For this reason, there is the so-called route diversity method which provides the main line and the backup line in different routes, that is, different optical fiber cables.
On the other hand, a maintenance signal such as a supervisory/order wire signal is required to make the necessary maintenance of the transmission system. The maintenance signal is inserted into or dropped from a main signal as overhead and is transmitted between stations via a main transmission line. Hence, if the transmission line is switched due to a failure, it is also necessary to switch the transmission line which is used to transfer the maintenance signal. The present invention relates to such a system for switching the maintenance signal transmission line.
FIG. 1 shows an example of a conventional transmission system in which a main line ML and a backup line BL coexist within a single optical fiber cable OFC. This type of optical fiber cable OFC includes 12 optical fibers, for example, and a part is used as the backup lines. For example, one optical fiber is used as a transmitting backup line for transmission and another optical fiber is used as a receiving backup line for reception. In FIG. 1, it is assumed that a terminal 1 is used as the transmitting station and a terminal 2 is used as the receiving station, and that a repeater 3 is provided between the terminals 1 and 2.
FIG. 2 shows a case where a maintenance signal, and particularly a telephone signal OW for maintenance, is transmitted in the transmission system shown in FIG. 1. At the transmitting station, the signal OW is split by a hybrid unit H and is transmitted to both the main line ML and the backup line BL. At the receiving station, the signal OW is received by switching a switch SW to become connected to the main line ML. If the main line ML fails, the signal OW is received by switching the switch SW to become connected to the backup line BL.
When the transmission system has the main and backup lines ML and BL provided within the same optical fiber cable, both the main and backup lines ML and BL fail if the optical fiber cable fails and this failure cannot be cured unless the optical fiber cable is repaired. But when the transmission system employs the route diversity method and the main and backup lines ML and BL are provided within mutually different optical fiber cables, the transmission system becomes as shown in FIG. 3. In other words, the main line ML and the backup line BL both connect the two terminals 1 and 2, but through different routes.
In FIG. 3, the main line ML and the backup line BL both connect the two terminals 1 and 2. However, because the routes taken by the main and backup lines ML and BL are different, the number of repeaters REP in the route, the length of the route and the like are not necessarily the same for the main and backup lines ML and BL.
Suppose that a failure occurs at a point "X" on the main line ML in FIG. 3 and the line being used is switched from the main line ML to the backup line BL. In this case, if the system shown in FIG. 2 is employed, a signal OW1 is transmitted to both the main and backup lines ML and BL from the terminal 1. But since the connection is switched from the main line ML to the backup line BL at the terminal 2, only the signal OW1 transmitted via the backup line BL is received by the terminal 2. The transmission of the maintenance signal between the terminals 1 and 2 is ensured by this system, however, the signal OW at a repeater REP.sub.1 of the main line ML causes a problem.
In other words, even if a failure occurs in a line, a transmission path for the maintenance signal between a repeater of this line and a terminal which is connected to this line must be maintained. In FIG. 3, the transmission path between the repeater REP.sub.1 and the terminal 2 is normal even though the failure exists at the point "X", and the transmission of the maintenance signal between the repeater REP.sub.1 and the terminal 2 must be maintained. However, since the switching is made at the terminal 2 to switch the connection from the main line ML to the backup line BL in response to the generation of the failure at the point "X", no transmission of the maintenance signal is possible between the repeater REP.sub.1 and the terminal 2.
In order to overcome the above described problem, it is conceivable to add the signals received from the main and backup lines ML and BL by an adder ADD as shown in FIG. 4 instead of switching the connection as shown in FIG. 2. According to this conceivable system, the maintenance signal can be transmitted between the repeater REP.sub.1 and the terminal 2 even if a failure occurs on the main line ML, as long as the transmission path between the repeater REP.sub.1 and the terminal 2 is normal. However, a problem occurs when transmitting the maintenance signal between the terminals 1 and 2 when no failure exists. In other words, the routes taken by the main and backup lines ML and BL are mutually different when the route diversity method is employed, and the transmission delay via the main and backup lines ML and BL inevitably differ even if the signal is simultaneously split at the transmitting station. As a result, when no failure exists on the main and backup lines ML and BL, signals which are the same but have mutually different phases are added in the adder ADD at the receiving station, thereby generating beat or cancelling of voice. Therefore, it becomes impossible to make a normal communication.
Accordingly, when the main and backup lines are provided and the route diversity method is employed, a problem occurs when a failure occurs on the main line and the line being used is switched from the main line to the backup line at the receiving station. The problem is that, since the receiving station is switched and connected to the backup line, no communication can be made between a repeater on the main line and the receiving station even if the transmission path between this repeater and the receiving station is normal. In other words, since the transmission path between the repeater and the receiving station is normal, a signal such as the alarm indication signal (AIS) is transmitted to this transmission path, but the receiving station cannot receive this signal.
On the other hand, if the signals received via the main and backup lines are added at the receiving station in order to overcome the above described problem, a normal communication cannot be made when no failure exists on the main and backup lines because of the inevitable difference between the transmission delays of the signals transmitted via the main and backup lines.